Optical soldering is a process in which lamp or laser energy is used to form solder connections between two materials. A light beam is focused at desired locations on the materials. The light beam heats the materials and any solder between them to form the solder connections. A plurality of solder connections are generally serially formed in optical soldering processes.
Optical soldering is attractive because it offers the promise of automation. Features of optical soldering, such as a locally applied heat source with an intensity that may be varied rapidly, lend themselves to automation. This is especially true of laser soldering. In order to exploit this capability for automation, it is necessary to monitor the formation of the solder joint. Typically, solder joint formation takes from 0.2 to 2.0 seconds in a laser soldering process.
Previously, single-element sensors which measure the amount of thermal radiation emitted by a solder joint have been used to monitor solder joint formation. Examples of this technique are found in U.S. Pat. Nos. 3,803,413 to Vanzetti et al., 4,481,418 to Vanzetti et al., and 4,657,169 to Dostoomian et al. The detected radiation values in these systems are compared to standards to yield information about the solder joint. In the Dostoomian patent, for example, the radiation values are used to determine the phase of the solder in a reflow soldering operation. This information is then used to manipulate the soldering process.
The information available from thermal radiation from the solder joint is limited, however. The available information depends on many factors, such as the emittance of the target and the background thermal radiation that is present. In addition, radiation detection systems require infrared optics and other specialized equipment.